The present application is cross-referenced to U.S. patent application Ser. No. 09/312,411, filed May 14, 1999, and entitled xe2x80x9cPROCESS PRESSURE MEASUREMENT DEVICES WITH IMPROVED ERROR COMPENSATION, which is hereby incorporated by reference in its entirety.
The present invention relates to process transmitters of the type used to monitor industrial processes. More specifically, the present invention relates to an analog to digital converter used in such a device in a technique which reduces or suppresses hystersis.
Transmitters are devices which are used to monitor the operation of industrial processes. For example, transmitters are used to measure process variables such as pressure, temperature, flow, conductivity and pH of a process and provide an output related to the measured process variable. Frequently, a transmitter is located at a remote location and transmits the measured process variable back to a central control room.
The pressure of a process fluid (both liquid or gas) can be used to determine various types of information about the process. For example, process pressure can be used to measure flow rate of process fluid through a restriction or the level of process fluid in a container. One type of pressure sensor used in process transmitters is a deflectable diaphragm. As pressure is applied to one or both sides of the diaphragm, the diaphragm deflects. The deflection of the diaphragm can be detected by measuring a change in a ratio of electrical capacitance related to the deflection. This capacitance ratio is converted into a digital format using an analog to digital converter. One specific type of analog to digital converter which can be used in process transmitters is known as a sigma-delta converter. A sigma-delta converter for use in a process transmitter is shown and described in U.S. Pat. No. 5,083,091, entitled CHARGE BALANCED FEEDBACK MEASUREMENT CIRCUIT which issued on Jan. 21, 1992 and is incorporated herein by reference. Sigma-delta analog to digital converters offer high accuracy with relatively low power requirements. They are thus well suited for use in process transmitters which are typically required to be powered from the same two-wire process control loop used to transmit the process variable.
One problem with typical prior art sensors is hysteresis in deflection of the diaphragm. This can lead to errors in pressure measurements. There is a desire to improve the accuracy of pressure measurements obtained using deflectable diaphragms and a sigma delta converter, and to do so in a manner that improves accuracy, is inexpensive and reduces power consumption.
The present invention provides a technique for suppression of hysteresis error in pressure measurements using a compensation electrode on a deflectable diaphragm and an efficient circuit for implementing a sigma delta converter. The circuitry is preferably implemented on a single integrated circuit and used in a diaphragm/sigma delta converter system to suppress hysteresis in pressure measurement for increased accuracy. In one example, a process pressure transmitter has a deflectable diaphragm and analog to digital (A/D) converter arrangement, the A/D converter uses a charge inverter configuration that reduces power consumption and can be fabricated on the same chip as the other A/D converter circuitry thus providing cost savings in manufacturing. In one aspect, the charge inverter includes a switch capacitor.